The aim of this research is to elucidate the control of differentiation of a multipotent stem cell, the interstitial cell (i-cell) of hydra. I have recently developed a powerful method for examining i-cell differentiation, which coupled with the simplicity of hydra allows me to explore this problem in a manner not available in other systems. The method involves disaggregating hydra into single cells and forming cell aggregates which will regenerate entire animals. The cell types can be separated and animals constructed of selected cell compositions. The i-cells will be placed in specific cellular environments and their patterns of differentiation studied. These patterns will be analyzed quantitatively by disaggregating the regenerated animal into separate cells. I can identify isotopically labelled i-cells even after five cell divisions. I will study i-cell differentiations into four types of nematocytes because: (1) each has highly specific morphological characteristics which appear early in differentiation; (2) stimuli from other cells regulate the differentiation of i-cells into nematocytes; (3) i-cell differentiation into nematocytes depends in part on position in the body column. The objectives of the proposal are to: (1) identify which cell types influence i-cell differentiation into nematocytes; (2) identify at which stages in the differentiation sequence these controls are exerted; (3) explore the nature of position-dependent i-cell yields nematocyte differentiation by altering the position of the i-cell in the body column.